Dynohubs are nifty electrical generators built into special bicycle hubs. They were made by Sturmey-Archer in England for several decades, and as of 2010, a new version has been introduced by Sturmey-Archer in Taiwan, though only in combination with a drum brake. (Generator hubs are also currently made by Schmidt, Shimano, Shutter Precision and others. Strictly speaking, those are not "Dynohubs" because "Dynohub" remains a Sturmey-Archer trademark.)

Generator hubs are fairly heavy -- the modern ones less so, thanks to rare-earth magnets and aluminum shells -- but they are totally silent and have no moving parts nor mechanical friction. They work by revolving a ring-shaped multi-pole magnet attached inside an over-sized hub shell around a stationary armature (coil) attached to the axle.

Advances in magnet technology allow modern hub generators to be smaller and lighter than the original Dynohub, yet more powerful. Lights also are more efficient, with halogen bulbs or light-emitting diodes, so that a modern generator system can put out several times as much light as a classic Dynohub system. The original Dynohub's power output was 1.8 watts at 6 volts, while that of most other bicycle generators is 2.4 or 3.0 watts. The headlamp sold with the original Dynohub was carefully designed to make the best use of its low power.

An original Dynohub nonetheless provides ample power for a modern LED lighting system. There are even LED bulbs now which fit the original Raleigh lights. The original headlight bulb is not only much less bright, it is very rare by now. An original Dynohub may still be found from time to time, built into the heavy, steel wheel of a Raleigh Industries three-speed -- usually 32 spokes for the front wheel with the GH6 Dynohub, or 40 for the rear wheel with the AG 3-speed hub. A quick test is to hold a key across the two terminals while spinning the wheel. If the Dynohub is good, it will spark.

Dynohubs are AC devices. Hub generator systems put out useful amounts of light at lower speeds than tire-driven generators, because the alternating-current frequency is low. The bulb lights up on the peak of each pulse of current. At low speeds, a Dynohub-powered light pulses visibly, while a light powered by a tire-driven generator goes dim.

As the speed of any bicycle generator increases, so does the frequency of the AC. This is convenient, because the generator's armature coil passes AC less efficiently as the frequency increases, substantially countering the tendency of the voltage to increase with speed. This inductive effect makes it possible for the generator to be used at a wider range of speeds than would otherwise be possible.

Like other bicycle generators, though, a Dynohub will fry incandescent bulbs if you go too fast. High speed cyclists still have to worry about cooking bulbs, unless using a modern lighting system with a voltage regulator.
A generator hub in a small wheel will produce more power at low speeds and is more prone to blow out bulbs at higher speeds. Some modern generator hubs are designed for use with small wheels, avoiding these problems.

[Sheldon, writing sometime around the year 2000, goes on to describe a primitive voltage regulator which he constructed a number of years before that. If you are going to build one, note that the Dynohub's rated output of 0.3 ampere requires a nicad or nickel metal hydride battery with a capacity of 10 ampere-hours or more (typically, a stack of 5 high-capacity nickel metal hydride D cells -- one example) to avoid overcharging with his setup in the "day" position.

A nickel-cadmium or nickel-metal hydride battery can also over-discharge, suffering permanent damage as the stronger cells drive power backwards through the weaker ones ("polarity reversal"). A"smart" system shuts off charging when the battery is full, and stops discharge at a safe level. Modern generator systems with a stay-on when stopped ("standlight") feature use an LED with a capacitor for storage, to avoid all these issues.

Front- and side-facing reflectors are nearly useless when the bicycle is moving, but they are effective when it is stopped, and are another effective backup to a generator system -- John Allen]

I used to have a Dynohub on a tandem, and the bulb consumption was unacceptable. I solved the problem (and some others) by running the Dyno's output through a full-wave bridge rectifier and then hooking the DC in parallel with a 6 volt (5 x 1.2v cell) nickel cadmium battery. This not only provided light when I was stopped, the Dyno would re-charge the nicads, and, when we went so fast that the voltage rose above 6 volts, the low internal resistance of the nicads sucked up the excess, gaining a bit of extra charge and saving the bulb.

The rectified output of the Dynohub was always connected to the lights. There was no way to turn the light off while in motion. It would have been easy enough to rig a switch for that purpose, but I didn't see the need. The Dynohub has _very_ low drag.
[The drag appears greater when the wheel is off the ground, because of original Dynohub had ow drag. The Schmidt generator has the lowest of all current models, and the new Dynohub, the highest. Still, you are unlikely to notice it while riding. -- John Allen]

I had a three way switch connecting the battery pack to the lights. In the "night" position, the nicads were in parallel with the rectified output of the Dynohub, as described above.

In the "day" position, the nicads were connected to the lights and Dyno through a diode (rectifier). This would allow the Dyno to charge the battery when it was going fast enough, but would not pass electricity the other way, so that the battery would not drain running the lights.

The "park" position completely disconnected the battery, because diodes are not perfect, and there is a slight drain that would discharge the nicads over a period of time.

Dynohubs are suitable for this type of set up because they do not use the bicycle frame as a ground. If you use a full-wave rectifier, you must keep the AC circuit separated from the DC circuit. Since most bicycle generator setups are made to use the frame as one of the wires in the circuit, this is a problem. To use full-wave rectification, either the generator or the lamps must be kept insulated from the frame. [The original] Dynohubs are already insulated from the frame, that is why they have two screw terminals and use twin-lead wiring.
[This is true of some but not all new hub generators.]

About rectifiers:

A "rectifier" is a device for converting AC (Alternating Current) into DC (Direct Current). The simplest kind of rectifier is a "diode", sort of a one-way valve for electricity. If you hook up a diode in series (as part of the circuit wire) with an AC source, it will only pass current half the time, when the AC is in its compatible direction. What comes through is pulsating DC. The problem with using a simple diode is that you are throwing away half of the electricity.

There is a simple circuit using four diodes, called a "bridge rectifier" that gets around this. It effectively flips the polarity around every half-cycle (an oversimplification, I know) and turns the AC into DC with negligible loss. You could solder four diodes together in the proper configuration, as I did, but these days you can just buy a ready-made "full-wave bridge rectifier". [Considering that the Dynohub operates at low voltages, the loss is significant when using silicon diodes -- 1.4 volts. If you use germanium diodes, it is only 0.4 volts. Fancier circuitry can get the loss much lower yet, but this is only a do-it-yourself project for a serious electronics hacker. -- John Allen.]

"THE WORLD'S FINEST CYCLE LIGHT"

The headlamp is of modern design and is the only cycle headlamp fitted with external screw-focusing. The front is securely fixed and is proof against rattle or detachment through vibration. A 3-inch diameter accurately made and highly polished parabolic reflector ensures a light of exceptional brilliance. The switch is placed below the lamp for weather protection and the connections are neatly made inside the lamp, thereby avoiding unsightly external terminals. The rear lamp is entirely sealed against moisture and its plastic red dome is visible at all angles from the rear, the bulb being of sufficient brilliance to be seen at a considerable distance. [The headlamp produced, a narrow but rather bright beam., though the nickel-plated reflector needed polishing occasionally. The taillamp has no focusing reflector and is feeble, even compared with more-modern ones that use the same0.6 watt incandescent bulb.-- John Allen]

The ultra - modern generator, which is covered by world patents, has no mechanical losses whatever and electrically is highly efficient, giving an output of 2 watts at 6 volts. [The bulbs used were rated at 1.8 watts -- John Allen] Using the existing wheel bearings and having no troublesome contact brushes it is entirely without mechanical friction or wearing parts, so that the effort to propel it is negligible. Being gearless it is absolutely silent, and its position in the hub protects it from damage. Voltage regulation is remarkably good, giving a good light at low speeds without an undue rise in voltage at the higher speeds, so guarding against the burning-out of bulbs.

Servicing the GH-6 Front Dynohub

Chris Hayes was kind enough to type up the instructions for dismantling and re-assembly of GH6 Dynohub from "The Secret is Fully Enclosed" the Sturmey Archer Catalogue for 1956.

[The Dynohub's bearing cups are integral with the hub shell, and can't be replaced. The bearing cones look like those for the AW hub, but they are not the same, as the axle diameter is smaller -- John Allen]

TO DISMANTLE THE GH6 HUB

Note: these instructions must be followed carefully. Separating the armature from the magnet will permanently weaken the magnet. -- John Allen]

Proceed as follows:

Remove the dynamo-side locknut and washers, making a note of their arrangement so that they can be replaced in their original positions.

Remove the four magnet-fixing nuts and lock washers from the back of the hub drum, and then remove the four magnet-fixing screws.

Hold the wheel, with the dynamo downwards, just above the work-bench. A few light taps with a mallet on the end of the spindle will cause the dynamo unit to drop out complete.

The magnet spacing ring can now be lifted out of the hub drum.

Unless it is essential to do so, the armature and magnet should never be separated. If they are to be separated, a keeper ring is absolutely necessary, because the magnet will lose some of its magnetism unless there is always iron inside it. Even a moment's separation will cause loss of magnetism, and a spanner placed across the magnet is useless as a substitute for a keeper ring. To separate the magnet and the armature, hold the dynamo unit in the left hand, with the terminal plate against the palm. Then fit a keeper over the armature and lightly tap the keeper with the palm of the right hand. The magnet will slide off the armature and on to the keeper. At this stage it is wise to test the armature with a test meter. If there is no reading on the test meter there is a break in the winding. If a test meter is not available, a battery and bulb may be connected as shown in the diagram, and if the bulb does not light a break in the armature winding is indicated. A second test is to disconnect the lead from one of the armature terminals and touch the outer edges of the armature with the bare lead. If the bulb lights, this indicates a short circuit.

Remove the cone locknut on the left-hand side and unscrew the left-hand cone.

The channel-section dust cap is just a press-in fit and can be prised out with a wide screwdriver (to avoid damage). The ball cage may be lifted out for examination of the hub bearing surface. If a new ball retainer and balls have to be fitted, it is usually best to fit a new dust cap as well. Both hub cups are part of the hub shell and if either is worn a complete new shell must be fitted.

The spindle may now be pulled out from the dynamo side,together with the right-hand cone. If the bearing surface is in good condition and the spindle threads are sound, there is no need to remove the cone.

The ball cage may be lifted out for examination of the hub bearing surface.

SPECIAL NOTE. - GH6 hubs prior to 1952 had the adjusting cone on the dynamo side. This cone is extended to pass through the armature body and is flatted at the outer end to take (K428) notched adjuster washer, by means of which the cone may be turned. Dismantling instructions from 1 to 5 remain exactly the same as for the current model. For paragraph 6 read 'Unscrew the dynamo-side cone and lift the ball cage out of the hub shell. The spindle may now be pulled out from the other side, together with the fixed cone.' All further comments apply equally to all GH6 hubs.

TO RE-ASSEMBLE THE GH6 HUB

Proceed as follows (see notes at end if hub was supplied before April 1952):

Fit the ball cage with the ball-retainer ring facing outwards, into the cup on the left-hand (the smaller) end of the hub shell. If a new ball-retainer is being fitted, the dust cap also should be new.

Fit the dust cap, with the channel facing outwards, and press it home (or tap lightly with a hammer).

If the dynamo-side cone has been removed from the spindle, replace it (LB357AZ) on the spindle and screw it up tight against the shoulder on the spindle.

Fit the ball cage, with the ball retainer ring facing outwards, into the cup on the dynamo-side of the hub shell.

Insert the spindle into the hub shell from the dynamo-side.

Fit the left-hand cone and adjust the hub bearings as described in The Fitting and Adjustment of Sturmey-Archer Hubs. (A correctly adjusted wheel must have a slight trace of play at the rim.)

Fit the cone locknut and screw it up tight against the left-hand cone.

If the magnet and armature have been separated, take the magnet and keeper ring in the left hand and, with the right hand, lay the armature alongside it.

While holding the magnet with the chamfer facing outwards, push the armature and the keeper through so that the magnet slides from the keeper on to the armature.

Parts Not Available

No replacement parts are available for GH6 Dynohubs. Please do not write to me asking for Dynohubs or parts; I don't have any, and I don't know where you can find any.
[If you want to rebuild one, have a spare on hand for parts. -- John Allen]